Modern agriculture, including silviculture, often requires the planting of large numbers of substantially identical plants genetically tailored to grow optimally in a particular locale or to possess certain other desirable traits. Production of new plants by sexual reproduction, which yields botanic seeds, can be slow and is often subject to genetic recombinational events resulting in variable traits in the progeny. Also, such crossing is time- and labor-intensive. Further, inbred strains such as those used to perform such crosses often lack vigor, resulting in low seed productivity.
Despite the drawbacks of conventional crossbreeding by sexual means, botanic seeds produced by such methods have an important advantage in that each seed comprises food-storage organs and protective structures that shelter the plant embryo inside the seed from the harsh soil environment and nurture the embryo during the critical stages of sowing and germination. Without such organs and structures, the plant embryo would be incapable of surviving in nature until it grew to seedling size.
In view of the disadvantages of producing large numbers of identical progeny plants by sexual means, propagation of commercially valuable plants via culturing of somatic or zygotic plant embryos has been intensively studied. Such "asexual" propagation has been shown for some species to yield large numbers of genetically identical embryos each having the capacity to develop into a normal plant. Unfortunately, these embryos, which are produced under laboratory conditions, lack the protective and nutritive structures found in seeds. As a result, the embryos must usually be further cultured under laboratory conditions until they reach an autotrophic "seedling" state characterized by an ability to produce their own food via photosynthesis, resist desiccation, produce roots able to penetrate soil, and fend off soil microorganisms. Such extensive laboratory culture during several distinct stages in plant development is time-consuming, resource-intensive, and requires skilled labor.
Some researchers have experimented with the production of "artificial" seeds in which individual plant somatic or zygotic embryos are encapsulated in a hydrated gel. (As used herein, "hydrated" denotes the presence of free water interspersed throughout the matrix of gel molecules comprising the gel capsule.) This method evolved from other work showing that encapsulating seeds in hydrated gels can improve germination in some species, especially since such gels can be supplemented with plant hormones and other compounds that aid germination and improve seedling survival in the field. With respect to artificial seeds, reference is made to European Patent Application No. 0,107,141 to Plant Genetics, Inc., published on May 2, 1984 (claiming priority under U.S. Pat. No. 4,562,663, filed on Oct. 12, 1982), teaching that hydrated gels used to encapsulate plant embryos should permit gas diffusion from the environment to the embryo and protect the embryo from abrasion. A suitable gel can be selected from alginates, guar gums, agar, agarose, gelatin, starch, polyacrylamide, and other gels. The gel can include additives such as plant nutrients, pesticides, and hormones. If necessary, the gel can be surface-hardened to confer further resistance to abrasion and penetration.
While a hydrated gel capsule seems to provide adequate moisture for a plant embryo and satisfactory protection against physical trauma in some instances, it has a poor permeability to atmospheric gases, especially oxygen, necessary for survival and growth of the embryo. As a result, there has been some effort directed to increasing the amount of oxygen inside the capsule. U.S. Pat. No. 4,808,430 to Kuono discloses encapsulating a seed in a hydrated gel along with an air bubble. Unfortunately, such a bubble actually contains a very small volume of air which in many instances does not provide enough oxygen for proper germination. This is especially the case when such bubble-containing capsules are stored for a length of time at room temperature. At room temperature, embryos of many types of plants respire, even if not actually germinating, which consumes oxygen. Since a hydrated gel is a poor absorber of atmosphere oxygen, the embryo in the seed soon becomes oxygen-starved despite a presumably initially adequate supply in the bubble. As a result, no oxygen is left after such storage to support germination.
The drawbacks of including an air bubble along with an encapsulated seed would not be fully rectified by encapsulating an embryo or seed in a foamed gel containing multiple air bubbles. The actual area available for gas exchange between the surrounding atmosphere, the gel capsule, the air bubbles, and the embryo is still quite small in a foamed gel. Such a small area, in combination with the low transfer rate of oxygen between air and a hydrated gel, would yield too low a rate of oxygen delivery to the embryo, especially during germination when oxygen requirements rapidly escalate.
Hence, there is a need for an analog of botanic seed comprising a plant embryo in contact with a hydrated gel having an elevated concentration of oxygen.